1. Field of the Invention
The present invention relates to an electrophotographic carrier and a developer using the carrier. In addition, the present invention relates to a developing method, an image forming apparatus, and a process cartridge using the developer.
2. Discussion of the Related Art
Developing methods for use in electrophotography are classified into a one-component developing method using a one-component developer consisting essentially of a toner and a two-component developing method using a two-component developer including a toner and a carrier such as a glass bead, a magnetic carrier, and a coated carrier, the surface of which is coated with a resin, etc.
Since the two-component developing method uses a carrier, which typically has a wide surface area so as to triboelectrically charge a toner, the two-component developing method has advantages over the one-component developing method in view of providing stable charging ability and producing high quality images for a long period of time. In addition, the two-component developing method has a high ability to feed a toner to the developing area. Therefore, the two-component developing method is widely used for digital electrophotographic systems in which an electrostatic latent image is formed on a photoreceptor by a laser beam and then made visible.
In order to respond to recent demands for improving resolution, highlight reproducibility and uniformity (granularity) of images, and colorization of images, a minimum unit composing a latent image, i.e., a dot, is modified to be much smaller and denser. There is a need for a developing system which can faithfully develop the latent image (i.e., dots), and various attempts have been made from aspects of improving both image forming process and developer (i.e., a toner and a carrier). From the aspect of image forming process, techniques of narrowing the developing gap, thinning layers of the photoreceptor, and making the diameter of the writing beam smaller are effective, but these techniques still have disadvantages of high cost and poor reliability.
On the other hand, from the aspect of developer, dot reproducibility is largely improved by using a toner having a small particle diameter. However, there is a problem that a developer including a toner having a small particle diameter tends to produce images with low image density and fog in that the background portion of an image is soiled with toner particles. When the toner is a full-color toner, a binder resin having a low softening point is typically used. Such a toner tends to adhere to the carrier, compared to a monochrome toner, and to deteriorate properties of the developer. As a result, toner scattering and fog in the background are easily caused.
Carriers having a small particle diameter have been also proposed.
For example, published unexamined Japanese Patent Application No. (hereinafter referred to as JP-A) 58-144839 discloses a magnetic carrier including a particulate ferrite having a spinel structure, and having an average particle diameter of less than 30 μm. Since this carrier is not covered by a resin and used for low electric field, the developed mass per area is poor and the life is short.
Japanese Patent No. (hereinafter referred to as JP) 3029180 discloses a carrier having a 50% average particle diameter (D50) of from 15 to 45 μm, and including carrier particles having a particle diameter of less than 22 μm in an amount of from 1 to 20%, carrier particles having a particle diameter of less than 16 μm in an amount of not greater than 3%, carrier particles having a particle diameter of not less than 62 μm in an amount of from 2 to 15%, and carrier particles having a particle diameter of not less than 88 μm in an amount of not greater than 2%, and further having a specific surface area.
Such a carrier having a small particle diameter has the following advantages.
(1) The surface area per unit volume is large. Therefore, the carrier is able to satisfactorily give charge to toner particles by friction, and therefore insufficiently-charged toner particles and reversely-charged toner particles are hardly produced. As a result, fog hardly occurs in the background and toner particles hardly scatter and blur, resulting in producing images with good dot reproducibility.
(2) For the above reason, the resultant image has high image density.
(3) Dense magnetic brushes can be formed. Since such a magnetic brush has good fluidity, the resultant image hardly has a brush mark.
On the other hand, spherical carriers, which have stable charging ability and are capable of producing uniform images, have been proposed. For example, JP 3078828 discloses a carrier including core particles which are spheroidized by a plasma treatment. Published Japanese translation of PCT international patent application No. 2005-524249 discloses a method for forming spherical particles from irregular particles by a plasma treatment.
However, as the particle diameter of a carrier decreases, the magnetic binding force thereof drastically decreases at a rate of cube of the particle diameter. As a result, a carrier particle or a cut magnetic brush tends to deposit on a latent image (this phenomenon is hereinafter referred to as “carrier deposition”), and make scratches on a photoreceptor or a fixing roller.